If 60Hz is the preferred rate, then why is the monitor telling the computer
that it should run at 75Hz? Surely *that* is the question. {Of course, it
could all just be due to a badly-shielded cable ..... that wouldn't really
be your fault if it were hard-wired at the monitor end, though.}

As far as I know, the communication protocol don't support
a "preferred rate". All it says is "this screen support from 60 to 75 Hz.",
as well as some other information. In the CRT days, the highest
supported refresh rate was what you wanted, so many a driver still
tries for that. With LCD you may want a lower rate, like in this case.
But drivers don't do that automatically.
Helge Hafting